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GEL POLYMER ELECTROLYTE BASED ON

N-PHTHALOYL CHITOSAN AND ITS APPLICATION IN DYE-SENSITIZED SOLAR CELLS

SITI NOR FARHANA BT YUSUF

THESIS SUBMITTED IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF

PHILOSOPHY

DEPARTMENT OF CHEMISTRY FACULTY OF SCIENCE UNIVERSITY OF MALAYA

KUALA LUMPUR

2017

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of Malaya

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ii UNIVERSITI MALAYA

ORIGINAL LITERARY WORK DECLARATION

Name of Candidate: SITI NOR FARHANA BT YUSUF (I.C. No:

Registration/Matric No: SHC120013

Name of Degree: DOCTOR OF PHILOSOPHY CHEMISTRY OF SCIENCE Title of Project Paper/Research Report/Dissertation/Thesis (“this Work”):

GEL POLYMER ELECTROLYTE BASED ON N-PHTHALOYL CHITOSAN AND ITS APPLICATION IN DYE-SENSITIZED SOLAR CELLS

Field of Study: POLYMER CHEMISTRY I do solemnly and sincerely declare that:

(1) I am the sole author/writer of this Work;

(2) This Work is original;

(3) Any use of any work in which copyright exists was done by way of fair dealing and for permitted purposes and any excerpt or extract from, or reference to or reproduction of any copyright work has been disclosed expressly and sufficiently and the title of the Work and its authorship have been acknowledged in this Work;

(4) I do not have any actual knowledge nor do I ought reasonably to know that the making of this work constitutes an infringement of any copyright work;

(5) I hereby assign all and every rights in the copyright to this Work to the University of Malaya (“UM”), who henceforth shall be owner of the copyright in this Work and that any reproduction or use in any form or by any means whatsoever is prohibited without the written consent of UM having been first had and obtained;

(6) I am fully aware that if in the course of making this Work I have infringed any copyright whether intentionally or otherwise, I may be subject to legal action or any other action as may be determined by UM.

Candidate’s Signature Date

Subscribed and solemnly declared before,

Witness’s Signature Date

Name:

Designation:

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ABSTRACT

It is widely known that chitosan is not soluble in common organic solvents.

Hence there is a need to increase its solubility in a wider range of solvents. To do this, the chitosan biopolymer has been modified by the process of phthaloylation to form N-phthaloyl chitosan (PhCh) by reacting phthalic anhydride with chitosan in dimethylformamide (DMF). The chitosan derivatives, PhCh, can dissolve in DMF, DMSO, DMAc and pyridine. Fourier transform infra-red (FTIR) and proton nuclear magnetic resonance (1H NMR) spectroscopies were used to confirm the PhCh formation and structure. The phthalimido and aromatic peaks of PhCh were seen at 1772, 1708 and 719 cm−1, respectively, and two sets of peaks from 1H NMR centered at 3.0 and 7.5 ppm verified that chitosan has been phthaloylated. The PhCh-based gel polymer electrolytes (GPE) consist of ethylene carbonate (EC), and DMF with different contents of tetrapropylammonium iodide (TPAI) and iodine. X-ray diffraction studies reveal that addition of tetrapropylammonium iodide (TPAI) further reduced the crystallinity of the PhCh. FTIR spectroscopy showed the interaction between polymer, plasticizer and salt. GPE comprising of PhCh : EC : DMF : TPAI : I2 in wt.% ratio of 12.0 : 36.1 : 36.1 : 14.4 : 1.4 exhibited the highest conductivity of 5.46 mS cm−1 at 30 ℃. When used in dye-sensitized solar cell (DSSC), it gave the best performance with the efficiency of 5.0 %, JSC of 12.72 mA cm−2, VOC of 0.60 V and fill factor of 0.66. To further improve the efficiency of the solar cell, lithium iodide (LiI) has been added to the PhCh-based electrolyte. The efficiency improved to 6.36 %, with the JSC of 17.29 mA cm‾2, VOC of 0.59 V and fill factor of 0.62.

Addition of 1-butyl-3-methylimidazolium iodide (BMII) ionic liquid to the electrolyte enhanced the DSSC efficiency to 6.69 % with the JSC of 16.53 mA cm‾2, VOC of 0.62 V and fill factor of 0.65.

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ABSTRAK

Secara meluas diketahui bahawa kitosan tidak larut dalam pelarut organik. Oleh itu, terdapat keperluan untuk meningkatkan kelarutan dalam julat yang lebih luas.

Untuk itu, biopolimer kitosan telah diubah suai dengan proses “phthaloylation“ untuk membentuk N-phthaloylchitosan (PhCh) dengan menindakbalaskan kitosan dengan acetic phthalic di dalam dimethylformamide (DMF). Derivatif chitosan, PhCh, boleh larut didalam DMF, DMSO, DMAc dan piridina. Fourier infra-merah (FTIR) dan proton resonans magnetik nuklear (1H NMR) spektroskopi telah digunakan untuk mengesahkan struktur PhCh. Puncak bagi phthalimido dan aromatik PhCh terdapat masing-masing pada 1772, 1708 dan 719 cmˉ1 dan dua set puncak dari 1H NMR berpusat di 3.0 dan 7.5 ppm mengesahkan pembentukkan PhCh. Elektrolit gel polimer (GPE) berdasarkan PhCh terdiri daripada etilena karbonat (EC), DMF, pelbagai kandungan tetrapropylammonium iodida (TPAI) dan iodin. Kajian pembelauan sinar- X menunjukkan bahawa penambahan tetrapropylammonium iodida (TPAI) terus mengurangkan crystallinity dalam PhCh. FTIR spektroskopi menunjukkan interaksi antara polimer, plasticizer dan garam. GPE yang terdiri daripada PhCh: EC: DMF:

TPAI: I2 dalam nisbah wt.% 12.0. 36.1: 36.1: 14.4: 1.4 menunjukkan kekonduksian tertinggi 5.46 mS cmˉ1 pada 30 ℃. Apabila digunakan dalam pewarna peka sel solar (DSSC), ia memberikan persembahan yang terbaik dengan kecekapan sebanyak 5.0%, JSC sebanyak 12.72 mA cmˉ2, VOC 0.60 V dan isi faktor 0.66. Untuk meningkatkan lagi kecekapan sel solar, lithium iodida (LiI) telah ditambah kepada elektrolit.

Kecekapan meningkat kepada 6.36%, dengan JSC 17.29 mA cmˉ2, VOC 0.59 V dan isi faktor 0.62. Dengan penambahan cecair ionik 1-butil-3-methylimidazolium iodida (BMII) kepada elektrolit, kecekapan DSSC terus meningkat kepada 6.69% dengan JSC

sebanyak 16.53 mA cmˉ2, VOC sebanyak 0.62 V dan isi faktor 0.65.

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ACKNOWLEDGMENTS

In the name of Allah, the Most Gracious, the Most Merciful.

I would first like to thank my thesis advisor Prof. Dr. Rosiyah Yahya and Prof Madya Dr. Siti Rohana Majid. Without their assistance and dedicated involvement in every step throughout the process, this thesis would have never been accomplished.

A million thanks also to both of my mentors, Prof Dr. Abdul Kariem Arof and Prof.

Dr. Mohamed Abdul Careem. Your encouragement and advice has led me to places I never thought I would go. Thank you so much for your support, understanding and mentorship throughout these past years.

To my fellow labmates, from both Chemistry (Vidhya, Danial & polymerlicious kakaks) and Physics departments (especially my ‘Swedish’ crew: Dr. Bandara, Hazirah & Fareezuan), seniors and juniors, thank you so much! Thanks for the fun and support! I cannot begin to express my gratitude and appreciation for their friendship. I am lucky to have made such great friends. I greatly look forward to having all of you as colleagues in the years ahead!!

Getting through my dissertation required more than academic support, and I have many, many people to thank for listening to and, at times, having to tolerate me over the past years. Most importantly, none of this could have happened without my family.

I must express my very profound gratitude to my parents, daddy & mommy, for giving birth to me in the first place and supporting me spiritually throughout my life. This accomplishment would not have been possible without them. I am also grateful to my siblings, Along, Abang and especially Adik who were always keen to know what I was doing and how I was proceeding, although it is likely that they have never grasped what it was all about! Thank you for the countless screams of joy whenever a significant momentous was reached and also just your general impudence. My beloved uncles, aunties and cousins especially McD, Kekra, Eppy & Wana. Thank you so much!

To my cheerleaders aka my best friends, Aqsa, Yomie, Jack, AJ, Yoyong, Kak Ina, Wawan, Kak Dini, thank you so much! Thank you for knowing exactly when to tell me what I want to hear, when I want to hear it the most. I just don’t know how I can say thank you to friends who understand all the things I never say, and never say anything I don’t understand. Also, thank you for removing the word EXPECTATION and adding the world HAPPINESS to the dictionary of our friendship!

Thank you so much.

With love, Farhana

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TABLE OF CONTENTS

ABSTRACT ... iii

ABSTRAK ... iv

ACKNOWLEDGMENTS ... v

TABLE OF CONTENTS ... vi

LIST OF FIGURES ... ix

LIST OF TABLES ... xiii

LIST OF ABBREVIATIONS AND SYMBOLS ... xiv

LIST OF APPENDICES ... xvi

CHAPTER 1 : INTRODUCTION ... 1

1.1. Motivation ... 1

1.2. Objectives of the present investigation ... 5

1.3. Scope of research work ... 6

1.4. Outline of the research ... 7

CHAPTER 2 : LITERATURE REVIEW ... 8

2.1. Introduction of Biopolymer ... 8

2.1.1. Chitosan ... 9

2.1.2. Modification of Chitosan ... 13

2.1.3. N-phthaloylation of chitosan ... 15

2.2. Solar cell ... 20

2.2.1. Dye-sensitized Solar Cell (DSSC) ... 20

Open-circuit voltage ... 23

Short-circuit current ... 24

2.2.2. Photo-Active Electrode ... 25

Mesoporous layer ... 25

Blocking layer ... 27

Dye as sensitizer ... 27

2.2.3. Counter Electrode ... 32

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2.2.4. Electrolyte for DSSC ... 32

Ionic liquid ... 33

Solid polymer electrolytes ... 34

Gel polymer electrolytes ... 35

Bulky cation ... 36

CHAPTER 3 : RESEARCH METHODOLOGY ... 39

3.1. Chemicals ... 39

3.2. Synthesis of N-phthaloylchitosan ... 39

3.3. Preparation of Gel Polymer Electrolytes ... 39

3.3.1. Gel polymer electrolytes with single salt ... 41

3.3.2. Gel polymer electrolytes with double salt ... 41

3.3.3. Gel polymer electrolytes with addition of ionic liquid ... 42

3.4. Characterisations of Gel Polymer Electrolytes ... 43

3.5. Solubility ... 43

3.6. Fourier Transformed Infra Red (FTIR) ... 43

3.7. Proton Nuclear Magnetic Resonance (1H NMR) ... 44

3.8. X-ray Diffraction (XRD) ... 44

3.9. Electrical Impedance Spectroscopy (EIS)... 44

3.10. Dye-Sensitized Solar Cell ... 46

3.10.1. Preparation of dye solution ... 46

3.10.2. Preparation of electrodes ... 46

3.10.3. Fabrication and characterisation of DSSC ... 47

CHAPTER 4 : RESULTS AND DISCUSSIONS ... 50

4.1. N-phthaloylchitosan of Chitosan ... 50

4.1.1. FTIR analysis ... 50

4.1.2. 1H NMR Analysis ... 53

4.1.3. XRD Analysis ... 54

4.1.4. Solubility ... 56

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4.2. Gel Polymer Electrolyte With Single Salt ... 59

4.2.1. EIS Analysis ... 59

4.2.2. FTIR Analysis ... 62

4.2.3. XRD Analysis ... 70

4.2.4. DSSC Analysis ... 71

4.3. Gel Polymer Electrolyte With Double Salts ... 75

4.3.1. EIS Analysis ... 77

4.3.2. FTIR Analysis ... 80

4.3.3. XRD Analysis ... 87

4.3.4. DSSC Analysis ... 88

4.4. Gel Polymer Electrolyte With Addition Of Ionic Liquid ... 92

4.4.1. EIS Analysis ... 94

4.4.2. FTIR Analysis ... 96

4.4.3. XRD Analysis ... 103

4.4.4. DSSC Analysis ... 103

CHAPTER 5 : CONCLUSIONS ... 106

5.1. Conclusions ... 106

5.2. Suggestions for future studies ... 107

REFERENCES ... 108

LIST OF PUBLICATIONS AND PAPERS PRESENTED ... 128

APPENDICES ... 129

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LIST OF FIGURES

Figure 1.1 : Energy resources 3

Figure 1.2 : Flow chart of current work 7

Figure 2.1 : Naturally occurring polysaccharides 9

Figure 2.2 : Structure of chitin and chitosan 10

Figure 2.3 : Preparation of chitin and chitosan from raw material (Alves & Mano, 2008)

10 Figure 2.4 : Formation of intra-molecular hydrogen bonds between

chitosan

13 Figure 2.5 : Multifaceted derivatization potential of chitin/chitosan

(Prashanth & Tharanathan, 2007)

15

Figure 2.6 : Phthaloylation of chitosan 16

Figure 2.7 : Structure of N- and O-phthaloylchitosan. 17 Figure 2.8 : X-ray diffraction diagrams of (A) fully deacetylated

chitosan, (B) PhCh prepared in DMF and (C) PhCh prepared in DMF/water (95/5) (Kurita et al., 2001)

18

Figure 2.9 : Dye-sensitized solar cell configuration 21 Figure 2.10 : Steps for generation of photocurrent in DSSCs 21 Figure 2.11 : Energy position of each component in DSSC 23 Figure 2.12 : UV–Visible spectra of certain ruthenium based dyes; (1)

N3 (dash), (2) N719 (solid) and (3) Z907 (dot) (Nosheen et al., 2016)

29

Figure 2.13 : Molecular structures of some ruthenium based sensitizer dyes

29 Figure 2.14 : Possible binding modes for carboxylic acid anchors onto a

metal oxide; (a) monodentate ester, (b) bidentate chelating, (c) bidentate bridging, (d) monodentate H-bonding, (e) bidentate H-bonding and (f) monodentate through C-O (Zhang et al., 2015)

31

Figure 2.15 : Relationship between efficiency and JSC with cation radius of six quaternary ammonium iodide in PAN based gel polymer electrolytes (Bandara et al., 2013)

38

Figure 3.1 : Chemical structure of DMF 40

Figure 3.2 : Photograph of the PhCh based gel polymer electrolytes 40

Figure 3.3 : Chemical structure of N3 dye 46

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Figure 3.4 : FTO glass after mesoporous layer of TiO2 was deposited 47 Figure 3.5 : Fabricated DSSC with PhCh gel polymer electrolytes 47 Figure 3.6 : Solar cell under illumination of 100 mW lamp 48

Figure 3.7 : Current-voltage curves of DSSCs 48

Figure 4.1 : FTIR spectra of (a) chitosan, (b) phthaloylchitosan 50 Figure 4.2 : Structure of (A) N-phthaloylated chitosan and (B) O,N-

phthaloylated chitosan

52 Figure 4.3 : 1H NMR spectra of phthaloylated chitosan 54 Figure 4.4 : XRD pattern for (a) chitosan and (b) phthaloylated

chitosan

55 Figure 4.5 : Disruption of hydrogen bonds after phthaloylation 55 Figure 4.6 : XRD analysis of (A) fully deacetylated chitosan, (B) PhCh

prepared in DMF, and (C) PhCh prepared in DMF:water (95:5) (Kurita et al., 2001)

56

Figure 4.7 : Effects of TPAI on the ionic conductivity of PhCh-EC- DMF based gel polymer electrolyte

60 Figure 4.8 : Temperature dependence of the ionic conductivity of the

PhCh-EC-DMF-TPAI gel polymer electrolytes

61 Figure 4.9 : FTIR spectra of (A) ethylene carbonate and (B)

dimethylformamide

63 Figure 4.10 : FTIR spectra of PhCh–EC–DMF–TPAI based gel polymer

electrolytes

64 Figure 4.11 : Deconvolution of individual FTIR regions: (A) ether

(1000–1200 cmˉ1); (B) amide (1580–1700 cmˉ1); (C) carbonyl (1700–1840 cmˉ1); and (D) amine/hydroxyl groups (3130–3700 cmˉ1)

65

Figure 4.12 : Relative FTIR band percentage area in the range of 1000 to 1200 cmˉ1

66 Figure 4.13 : Relative FTIR band percentage area in the range of 1580

to 1700 cmˉ1

67 Figure 4.14 : Relative FTIR band percentage area in the range of 1700

to 1840 cmˉ1

68 Figure 4.15 : Relative FTIR band percentage area in the range of 3130

to 3700 cmˉ1

70 Figure 4.16 : XRD patterns of PhCh based GPE with various content of

TPAI

71

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Figure 4.17 : Current–voltage curves for DSSCs based on PhCh–EC–

DMF–TPAI gel polymer electrolytes with varying amounts of TPAI

72

Figure 4.18 : Relationship between ionic conductivity and DSSC efficiency with the various mass of TPAI

74 Figure 4.19 : The two types of cations present in the PhCh–EC–DMF–

TPAI-Li GPE system

77 Figure 4.20 : Variation of activation energy values and conductivity

values as a function of LiI content

78 Figure 4.21 : Number density and ionic mobility of GPEs with different

ratios of LiI:TPAI

79 Figure 4.22 : FTIR spectra for PhCh based gel polymer electrolytes

containing various ratio of TPAI:LiI

81 Figure 4.23 : A graphical representation of the cation coordination to the

electron rich moieties in the GPE system

82 Figure 4.24 : Relative FTIR peak area for each deconvoluted peak in the

ether region

83 Figure 4.25 : Relative FTIR band percentage area in the range of 1580-

1700 cm−1

85 Figure 4.26 : Relative area percentage of the deconvoluted peaks in the

amide region

86 Figure 4.27 : Relative FTIR band percentage area in the range of 3130-

3700 cm−1

87 Figure 4.28 : XRD patterns of PhCh based gel polymer electrolytes with

various ratio of TPAI:LiI content

88 Figure 4.29 : Current-Voltage curve of PhCh-EC-DMF electrolytes with

various ratio of TPAI:LiI

90 Figure 4.30 : Effects of ionic conductivity and efficiency to the different

ratio of TPAI:LiI in double salt system

91 Figure 4.31 : Chemical structure of BMII ionic liquid 93 Figure 4.32 : Effects of BMII on the PhCh-EC-DMF-TPAI-LiI based

GPE

95 Figure 4.33 : Relationship between n and μ of the PhCh-EC-DMF-

TPAI-LiI based gel polymer electrolyte with various weight percentage of BMII

96

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Figure 4.34 : FTIR spectra of pure BMII ionic liquid 97 Figure 4.35 : FTIR spectra for PhCh based gel polymer electrolytes

containing various wt.% of BMII

98 Figure 4.36 : Relative FTIR percentage area in the region of 1000-1200

cmˉ1

100 Figure 4.37 : Relative FTIR percentage area in the region of 1580-1700

cmˉ1

101 Figure 4.38 : Relative FTIR percentage area in the region of 1700-1840

cmˉ1

101 Figure 4.39 : Relative FTIR percentage area in the region of 3130-3700

cmˉ1

102 Figure 4.40 : XRD pattern of the PhCh-EC-DMF-TPAI-LiI based GPE

with different wt. % of BMII

103 Figure 4.41 : J-V curve of the GPEs with various content of BMII 105

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LIST OF TABLES

Table 2.1 : Applications of chitosan in various fields 11 Table 2.2 : Examples of Chitosan-based polymer electrolyte 12

Table 2.3 : Applications of phthaloylchitosan 18

Table 2.4 : List of the DSSCs using natural dyes 30

Table 2.5 : Comparison of performance parameters of some

biopolymer electrolyte based DSSCs in recent literature

37 Table 3.1 : List of materials used throughout this work 39 Table 3.2 : Composition of electrolytes with various mass of TPAI 41 Table 3.3 : Composition of electrolytes with various ratio of

TPAI:LiI

42 Table 3.4 : Composition of electrolytes with various wt.% of BMII 42 Table 3.5 : Relation for ideal bulk electrical elements 45 Table 4.1 : Significant wavenumbers exhibited by N-phthaloylated

chitosan

51 Table 4.2 : Solubility of PhCh in various solvents 58 Table 4.3 : Ionic conductivity value of gel polymer electrolytes with

various content of TPAI at room temperature

61 Table 4.4 : J-V parameters of DSSC with various content of TPAI 73 Table 4.5 : Comparison of performance parameters of some DSSCs

in recent literature for electrolytes consisting of single and double salt systems

76

Table 4.6 : σ and Ea of GPEs with various mass ratios of TPAI and LiI

78 Table 4.7 : n, μ and D values for the GPEs with different ratios of of

LiI:TPAI

79 Table 4.8 : DSSC parameters of PhCh-EC-DMF-TPAI-LiI

electrolytes

90 Table 4.9 : Ionic conductivity values of PhCh-EC-DMF-TPAI-LiI

based gel polymer electrolyte with various weight percentage of BMII

95

Table 4.10 : Peak assignments of pure ionic liquid BMII 97 Table 4.11 : DSSC parameters of the GPEs with various content of

BMII

104

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LIST OF ABBREVIATIONS AND SYMBOLS

ATR Attenuated Total Reflectance

BMII 1-butyl-3-methylimidazolium iodide

CB Conduction band

DMAc N,N-dimetylacetamide DMF N,N-dimethylformamide DMSO Dimethyl sulfoxide DSSC Dye sensitized Solar Cell Ea Activation energy

EC Ethylene carbonate

EIS Electrical impedance spectroscopy FF Fill factor

FTIR Fourier Transform Infrared Spectroscopy FTO Fluorine Tin Oxide

GPE Gel polymer electrolyte

HOMO Highest occupied molecular orbital IL Ionic liquid

JSC Short circuit photocurrent density LUMO Lowest unoccupied molecular orbital Mw Weight average molecular weight N719 & N3 Ruthenium dye

NMR Nuclear magnetic resonance spectroscopy PC Propylene carbonate

PDI Polydispersity index

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PEO Polyethylene oxide PhCh N-phthaloylchitosan PP Polypropylene

PTFE Polytetrafluoroethylene PVC Polyvinyl chloride SPE Solid polymer electrolyte TBP Tert-butyl pyridine

Tg Glass transition temperature TGA Thermogravimetric analysis TPAI Tetrapropylammonium iodide

VB Valence band

VOC Open circuit voltage XRD X-ray diffraction

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LIST OF APPENDICES

Appendix A : Photographs of selected steps in preparation of PhCh.

Appendix B : Data analysis from the XRD curves of PhCh-EC-DMF- TPAI GPE.

Appendix C : Properties of XRD spectrum of PhCh based gel polymer electrolytes.

Appendix D1 : FTIR spectra for the deconvolution peaks at ether region for various content of TPAI.

Appendix D2 : FTIR spectra for the deconvolution peaks at amide region for different content of TPAI in the C=O region; (A) 1580 to 1700 cmˉ1 and (B) 1700 to 1840 cmˉ1.

Appendix D3 : FTIR spectra for the deconvolution peaks at N-H/O-H region for different content of TPAI in the PhCh based electrolytes.

Appendix E1 : FTIR spectra for the deconvolution peaks at ether region for various ratio of TPAI:LiI in the PhCh based electrolytes.

Appendix E2 : Deconvoluted peaks of PhCh-EC-DMF-TPAI-LiI in the (A) 1580-1700 cmˉ1 and (B) 1700-1840 cmˉ1 region.

Appendix E3 : FTIR spectra of the deconvolution peaks for various ratio of TPAI:LiI in the PhCh based electrolytes at 3500-3700 cmˉ1 region.

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1

CHAPTER 1 : INTRODUCTION

1.1. Motivation

Numerous polymers have been used as the host to ionic conduction. However, most of the polymers are synthetic. Examples are polyethylene oxide (PEO) (Das &

Ghosh, 2015; Karan et al., 2008; Karmakar & Ghosh, 2012), polystyrene (PS) (Rohan et al., 2014), polytetrafluoroethylene (PTFE) (Jeong et al., 2016; Mack et al., 2016; Rofaiel et al., 2012) and polyvinylchloride (PVC) (Ramesh & Arof, 2000, Ramesh & Arof, 2001;

Ramesh et al., 2007; Ramesh et al., 2002a). Synthetic polymers are detrimental to the environment as it is costly to recycle and has poor degradability, eventually finding its way into the ground soil and as far as the oceans as toxic waste pollutants.

It is these problems that have motivated researchers to turn towards biopolymers.

Among many potential biopolymers, chitosan is of particular interest as it exhibits a polyelectrolyte nature due to the protonated NH2 amino group in its backbone (Klotzbach et al., 2006; Payne & Raghavan, 2007; Wan et al., 2003). Chitosan is derived from hydrolysis of acetamide groups through alkaline treatment of chitin, the second most abundant natural polymer. However, chitosan is soluble only in dilute acidic solutions but not in organic solvents. For the purpose of electrochemical devices with metal components, usage of aqueous media would limit the lifespan and usability of the device due to corrosion of the metal parts. Thus, it has become a necessity for chitosan to be modified to meet the requirements of non-aqueous solvent compatibility.

In this work, chitosan has been modified using phthalic anhydride for N- phthaloylchitosan (PhCh) production. N-phthaloylchitosan is soluble in DMF, dimethylacetamide (DMAc), dimethylsulfoxide (DMSO) and pyridine (Kurita et al., 2003; Kurita et al., 2001; Kurita et al., 2005; Yoksan et al., 2001). The ability to dissolve in organic solvents is due to the presence of a new hydrophobic phthaloyl group along

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the chitosan backbone that prevents the formation of hydrogen bonding between the solvents and the amino and hydroxyl groups in chitosan.

Chitosan has previously been used as a host for ionic conduction but in the form of solid polymer electrolytes (Khiar et al., 2006; Majid & Arof, 2005; Osman & Arof, 2003). Solid polymer electrolytes have good mechanical strength, electrochemical stability, and ease of fabrication into devices. However its main drawback is that its ionic conductivity is not as high enough to be used in some application such as batteries. Before the emergence of solid polymer electrolytes, liquid electrolytes are used in electrochemical devices. Although liquid electrolytes have the advantage of higher ionic conductivities over solid polymer electrolytes, it does possess some weaknesses: it is prone to leakage, evaporation of the solvents, corrosion and electrochemical instability at high temperatures, all of which does not favour applications in devices and can harm the environment.

Stepping up to the challenge of combining the best properties from both solid and liquid electrolytes and without the weaknesses of either, many researchers have paved the way to arrive at a new class of material: the gel polymer electrolytes (GPE). The GPE can be considered as a liquid electrolyte trapped inside a polymer matrix and has conductivity that is liquid-like ionic (Arof et al., 2014a). Several polymer hosts have been used in the fabrication of GPE based DSSC such as polyaniline (PAN) (Arof et al., 2013; Bandara et al., 2010a; Bandara et al., 2010b; Bandara et al., 2013; Dissanayake et al., 2002;

Dissanayake et al., 2012), polyethylene oxide (PEO) (Bandara et al., 2012), polyvinyl acetate (PVA) (Arof et al., 2014b; Aziz et al., 2014), and poly(vinylidene fluoride) (PVdF) (Arof et al., 2014a).

However, this work looks into the potential of PhCh as a host for GPEs. In order to test the strength of the PhCh-based gel polymer electrolytes, it has been used as an electrolyte in DSSC. Why solar cell? This is because energy consumption is increasing

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from year to year and with rapid modernization the technology is becoming increasingly sophisticated technology (Maçaira et al., 2013). Energy resources can be divided into two parts, namely non-renewable and renewable as shown in Figure 1.1.

Figure 1.1: Energy resources.

Much of the electricity we currently use is generated from fossil fuels such as coal, oil and natural gas. Many issues arise from this matter, among which is the shortage of this non-renewable resource due to its high demand and usage. In addition, there is no solid guarantee against these resources since they are supplied by limited countries and its price in the market is also volatile. Several of the most well-known harmful gases released into the environment originates from the combustion of fossil fuels. These are nitrogen dioxide (NO2), sulphur dioxide (SO2), carbon monoxide (CO) and carbon dioxide (CO2). NO2 and SO2 which are acidic gases will dissolve in the rain to form acid rain that can damage buildings, plants and kill aquatic life. CO will react with haemoglobin in the blood and prevents it from carrying oxygen around the body resulting

Energy Resources

Non-renewable

Fossil Fuels;

Oil, Coal,Natural Gas Metallic Minerals;

Iron, Copper, Aluminium Nonmetallic Minerals;

Salt,Phosphates

Renewable

Solar Wind Water,Tides

Soil

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in oxygen starvation. Initially, CO2 is useful to us where it traps heat around the earth.

However, earth experiences exceptional warmth due to the excessive CO2 released into the environment. This leads to climate change, such as the melting of ice at the poles, sea water levels rising and experts predict more storms, drought, floods and famine.

In order to cut back on the amount of CO2 released and thus slow down global warming, another alternative energy source such as nuclear energy has been introduced.

Nuclear energy has two big advantages in that it gives out huge amounts of energy; a pellet of nuclear fuel the size of a pea can give as much energy as a tonne of coal and no CO2 or other greenhouse gases are produced. However, this energy source is based on nuclear fission which produces dangerous unstable atoms or radioisotopes. An explosion in a nuclear power station could pollute a large area with radiation. Nuclear energy is unfortunately not renewable.

Therefore, much effort is focused on renewable energy resources which mankind will depend more on in the future. Thus, there is a push to switch to a clean, free and extremely reliable source of power i.e. solar energy. A solar cell is a device that directly converts sunlight into electrical energy through the photovoltaic process. There are many types of solar cells, namely, silicon solar cell, perovskite solar cell, cadmium telluride solar cell, organic solar cell, quantum dot solar cell and dye-sensitized solar cell (DSSC).

Among many kinds of solar cells, DSSC has been selected for this research because it has several advantages such as it can achieve high sunlight to electrical energy conversion efficiency with low cost and is easy to fabricate. The first DSSC was introduced by O'Regan & Gratzel (1991) and is made up of three main components, namely, the photoactive electrode, electrolytes and a Pt counter electrode.

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In this work, the effect of double salts and ionic liquid in the GPEs on the cell parameters was investigated, with the goal to improve the efficiency of the DSSC. Iodide salts with bulky cations will be utilized. The bulky cations are expected to reduce cationic conductivity and thereby enhance the iodide ion conductivity and transference number in the electrolytes. In order to enhance the efficiency of the DSSCs, a mixture of two iodide salts has been applied in the gel polymer electrolytes. The mixed iodide salts one of bulky cation and the other small cation were used to supply the required iodide ion conductivity.

The presence of the small cations with high charge density are expected to contribute towards better photo-generation of electrons and their faster transfer across the dye-TiO2

interface (Arof et al., 2014a). Other efforts that has been done to increase the efficiency of the DSSC is by adding small portions of ionic liquids in the solid polymer electrolytes (Singh et al., 2011). In the present study, the effects of single cation, mixed cations and addition of BMII in PhCh-based GPEs in the improvement of DSSC performance was explored.

1.2. Objectives of the present investigation

1. To improve the solubility of chitosan in polar aprotic solvents by modifying it via phthaloylation process.

2. To produce a highly efficient DSSC by optimizing the PhCh-based gel polymer host using various masses of tetrapropylammonium iodide (TPAI).

3. To investigate the effects of mixed cation salts system in GPEs towards the efficiency of the DSSC.

4. To optimize the weight percentages of BMII ionic liquid to be added in the mixed cation GPEs in order to further enhance the efficiency of the DSSC.

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1.3. Scope of research work

The progress on phthaloylchitosan, quasi-solid polymeric ionic conductors and dye-sensitized solar cells are reviewed in Chapter 2. Chapter 3 will discuss the experimental procedures for the (i) modification and verification of chitosan and (ii) characterisation of the PhCh gel polymer electrolytes (GPEs). This chapter ends with the fabrication of DSSCs using the PhCh based electrolytes and cis-bis(4,4’-dicarboxy-2,2’- bipyridine)dithiocyanato ruthenium(II) (N3) dye as the sensitizer. Chapter 4 presents the results obtained from this study. This chapter comprises of four parts. The first one is the verification of the modified chitosan structural and its physical properties including its crystallinity and solubility. The second part discuss the phthaloylchitosan as the polymer host in GPE. The effect of the salts on dye sensitized solar cell efficiency, conductivity behavior and polymer-salt interaction will be discussed in this chapter. The third part includes the effects of mixed cations in the gel polymer electrolyte towards the DSSC.

The last part of this chapter discusses the results of introducing small amounts of ionic liquid to the GPE in order to further improve the efficiency of DSSC. The work will be concluded in Chapter 5. The flow chart in Figure 1.2 summarizes the current work.

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1.4. Outline of the research

Figure 1.2: Flow chart of current work.

Chitosan

N-Phthaloylchitosan (PhCh)

Characterization

•FTIR

1H NMR

•XRD

•Solubility

PhCh-Tetrapropylammonium iodide (TPAI)

Characterization

•Dye-sensitized solar cell

•Ionic Conductivity

•FTIR

•Crystallinity

PhCh-TPAI-Lithium iodide (LiI)

Characterization

•Dye-sensitized solar cell

•Ionic Conductivity

•FTIR

•Crystallinity

PhCh-TPAI-LiI-1-butyl-3- methylimidazolium iodide (BMII)

Characterization

•Dye-sensitized solar cell

•Ionic Conductivity

•FTIR

•Crystallinity

Modification of chitosan

Effects of ionic liquid Effects of mixed cations Effects of single salt

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CHAPTER 2 : LITERATURE REVIEW

2.1. Introduction of Biopolymer

A polymer is a substance that contains large molecules that is formed from many small molecules or monomers joined together. There are two types of polymers, natural and synthetic polymers. Examples of synthetic polymers include polyethylene oxide (PEO), polystyrene (PS), polyamide (PA) and polyvinylchloride (PVC). These show some good properties such as being light-weight, strong and can be molded into shape without breaking. However, this type of polymer which is mostly made from chemicals found in the naptha fraction of oil is unreactive as they are not affected by air, water, acids or other chemicals. This matter creates a problem since they do not break down or rot away, thus it will be difficult to decompose and resulting it to be costly to recycle.

Hence, the use of natural polymers especially biopolymers has re-emerged in the industry (Chaisorn et al., 2016; de Léis et al., 2017; Kim et al., 2017). Biopolymers have two significant advantages which are (1) it is a renewable resource and (2) it is biodegradable. Since nature has been busy producing natural polymers for millions of years, it has existed abundantly: carbohydrate is an example. Carbohydrate is an important naturally occurring substances that can be found in plants and animals. Figure 2.1 summarizes the classifies carbohydrates into simple and complex carbohydrates.

Simple carbohydrates consist only of monosaccharides, which is single sugar such as glucose, C6H12O6. By linking together two sugar units, for example, glucose and fructose, C6H12O6, disaccharide sucrose, C12H22O11, is obtained. Polysaccharides contain a large number of monosaccharide units joined together by glycosidic linkages.

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Figure 2.1: Naturally occurring polysaccharides.

2.1.1. Chitosan

Chitosan, a linear amino polysaccharide, has attracted the hearts of many researchers among the multitude of polysaccharides due to it being non-toxic, non-immunogenic, enzymatically biodegradable and biocompatible in animal tissues (Dodane & Vilivalam, 1998; Sashiwa et al., 2002). Chitosan comprises the repeating unit of β-(1-4) linked 2- amino-2-deoxy-D-glucopyranose and have no or small amounts of N-acetyl-D- glucosamine units (Badawy et al., 2004; Liu et al., 2005). Chitosan is a product of N- deacetylation of chitin when it is able to dissolve in dilute acids. The changes in the structure of chitin into chitosan can be seen from Figure 2.2. Chitin is the second most abundant natural polymer in the world after cellulose (Binette & Gagnon, 2007). It is widespread in the outer shells of insects (scorpions, ants, cockroaches, spider and beetles) and sea animals (annelid, mollusca coelenterate and crustaceans like crab and shrimp).

Carbohydrates

Simple carbohydrate

Monosaccharides

glucose

fructose

galactose

ribose

Complex carbohydrate

Disaccharides

sucrose

lactose

maltose

Polysaccharides

cellulose

Chitin/chitosan

amylopectin

amylose

alginic

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Other sources of chitin and chitosan is from microorganisms such as algae, yeast, mycelia penicillium, spores and also in the cell wall of certain fungi (Aranaz et al., 2009). Figure 2.3 shows the steps of extraction of chitin and chitosan from the raw materials.

O O

O OH

HO

HO O

HO NH

NH

O CH3

O CH3

O O

O OH

HO

HO O

HO NH

NH

O CH3

O CH3

O O

OH

HO NH O

CH3

n Chitin

O O

O OH

HO

HO O

HO

NH2

NH2

O O

O OH

HO

HO O

HO NH

NH2 O

CH3

O O

OH

HO

NH2

Chitosan n

Figure 2.2: Structure of chitin and chitosan.

Figure 2.3: Preparation of chitin and chitosan from raw material (Alves & Mano, 2008).

Chitosan is a material with high potential and have been used in various fields such as pharmaceutical, cosmetic, biomedical, environmental, agricultural,

Crustacean Shell

Washing and grinding

Deminera- lization/

HCl

Deproteini -zation/

NaOH Extraction

with acetone and drying Bleaching/

NaOCl

washing and drying

CHITIN

Deacetyla- tion/

NaOH

Washing and drying

CHITOSAN

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biotechnological, food industries and catalysts. The details have been tabulated in Table 2.1.

Table 2.1: Applications of chitosan in various fields.

Field Descriptions References

Pharmaceutical Chitosan has been used in pharmaceutical and drug delivery applications. Its absorption- enhancing controlled release and bioadhesive properties have made it useful for such applications.

Dodane &

Vilivalam (1998)

It has the potential to significantly improve the transmucosal delivery of macromolecule drugs.

Issa et al. (2005) Cosmetic Skin (moisture, treat acne), hair and oral care

(toothpaste and chewing gum).

Rinaudo (2006) Biomedical Chitosan plays a role in tissue engineering and

antimicrobial agents in wound healing applications.

Aranaz et al.

(2009) Environmental Chitosan flocculation involved in the removal

of phytoplankton cells from aquaculture systems to reduce the nitrogenous waste and improves water quality.

Lertsutthiwong et al. (2009) Chitosan can remove suspended titanium

dioxide particles in water by flocculation in the presence of humic acids.

Divakaran &

Pillai (2004)

Water treatment. Miretzky &

Cirelli (2009) Agricultural Chitosan can be used as a growth promoter. El-Sawy et al.

(2010)

Biotechnological Wool fabric. Issa et al. (2005) Food industries Antimicrobial properties of chitosan blends

with gliadin proteins isolated from wheat gluten.

Fernandez-Saiz et al. (2008) Catalysts Renewable polymeric supports for catalysts Macquarrie &

Hardy (2005)

Chitosan as Polymer Electrolytes

Chitosan has been used to develop high conducting polymer electrolyte systems as it has polyelectrolyte behaviour, a protonated amino group in its structure (Hu et al., 2007; Klotzbach et al., 2006; Payne & Raghavan, 2007; Wan et al., 2003). Moreover, chitosan attracts more attention with its chelating properties with various substances, such as fats, metals, proteins, and others (Bordenave et al., 2008). Polyelectrolyte complexes

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of chitosan with other natural sources such as DNA and proteins have also been reported.

Denuziere et al. (1998) had studied the chitosan polyelectrolyte complexes with polysaccharides, including those with glycosaminoglycans (GAG). In the field of chitosan polyelectrolyte complexes with synthetic polyacid anions, the largest numbers of publications were devoted to chitosan polyelectrolyte complexes with polyacrylic acid (PAA) (de la Torre et al., 2003; Shieh & Huang, 1997).

Polyelectrolyte is slightly different from solid or gel electrolyte because in polyelectrolyte, cationic or anionic groups are chemically bonded to a polymer chain, while their counterions are solvated by a high dielectric constant solvent and mobile. In polymer electrolytes, interaction of polymer with the doping salt will lead to the complexation. Chitosan has good complexing ability as the -NH2 groups are involved in specific interactions with metal ions (Rinaudo, 2006).

Table 2.2: Examples of Chitosan-based polymer electrolyte.

Chitosan Salt/acid References

Chitosan in acetic acid Lithium acetate Yahya & Arof (2002) Chitosan in acetic acid Sodium Alginate Smitha et al. (2005) Chitosan in acetic acid Sulfuric acid Smitha et al. (2008) Chitosan in acetic acid Ammonium nitrate Ng & Mohamad (2006) Chitosan in acetic acid Phosphoric acid Majid & Arof (2007) Chitosan in acetic acid Sodium alginate Sæther et al. (2008)

According to Baril et al. (1997), the four factors for the formation of complexes are (i) high concentration of polar solvating groups (-O-, -OH,-NH, -CN-), (ii) the donor number and polarizability of the solvating groups, (iii) low lattice energy of the doping salt and (iv) low lattice energy of the polymer. Although polyelectrolyte is a bit different from the polymer electrolyte, with all the polyelectrolyte features that exist in chitosan, it helps to fulfil the objective of the work to develop polymer electrolytes. Some examples of chitosan as polymer electrolytes host are shown in Table 2.2.

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2.1.2. Modification of Chitosan

Although it has a high potential as a base for polymer electrolyte, there is still a clear weakness in chitosan. Chitosan is reported to be only soluble in dilute aqueous acidic solution (pH 6.5) but insoluble in water and organic solvent (Aranaz et al., 2009; Pillai et al., 2009; Qin et al., 2006; Sashiwa et al., 2002). This is due to the free protonable amino groups present in the D-glucosamine units (Aranaz et al., 2009; Holappa et al., 2004).

The β-1,4’-glycosidic linkages give the biopolymer its rigid and crystalline structure besides promoting formation of intra-molecular hydrogen bonds (Bruice, 2004), involving the hydroxyl groups as shown in Figure 2.4. Solubility of chitosan is a very difficult parameter to control as it is related to the degree of acetylation, the ionic concentration, the pH, the nature of the acid used for protonation and the distribution of acetyl groups along the chain, as well as the conditions of isolation and drying of the polysaccharide (Rinaudo, 2006).

O O

O O

O

O O

O

N

N

O O

O O

O

O O

O

N

N

O CH3

O O

O

O

N

n

H

H

H

H

H

H

H

H

H

H

H

H H

H

H H H

H H

Figure 2.4: Formation of intra-molecular hydrogen bonds between chitosan.

The insolubility of chitosan in many common organic solvents limits its usage and leads to some disadvantages such as poor extent of reaction, structural ambiguity of the products and partial degradation due to harsh reaction conditions (Jančiauskaitė &

Makuška, 2008; Kurita, 2006).

With great potential in a variety of applications as well as abundant in existence on this earth, chitosan should be modified to fully explore its ability and development. In

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addition, the presence of two functional groups in chitosan’s backbone, NH2 and OH, enables modifications can be done. Many researchers have modified chitosan to fulfil the requirements in their respective fields (Inta et al., 2014; Jeon & Höll, 2003; Mourya &

Inamdar, 2008; Sobahi et al., 2014). For example, Sashiwa et al. (2002) reported that chemical modification of chitosan was necessary to improve its adhesion to plastics as well as its organosolubility so as to be able to adhere on the surface of the plastics which is hydrophobic. Roberts & Wood (2001) working in the development of chitosan-based anti-felting treatment for wool, increased the hydrophobic character of the chitosan by introducing long-chain N-acyl groups through reaction with hexanoic anhydride or similar long chain acid anhydride.

Various possible methods have been carried out to modify chitosan (Zohuriaan- Mehr, 2005) and some of the potential method is displayed in Figure 2.5 (Prashanth &

Tharanathan, 2007). Among the methods are phosphorylation (Ma et al., 2010; Wang et al., 2001), sulfonation (Fredheim & Christensen, 2003; Lv et al., 2014; Wolfrom & Han, 1959), xanthation (Sankararamakrishnan et al., 2006; Sankararamakrishnan & Sanghi, 2006), acylation (Peesan et al., 2006; Peesan et al., 2005; Wu et al., 2004; Zong et al., 2000), cross-linking (Bodnar et al., 2006; Maya et al., 2014), graft copolymerization (Aranaz et al., 2009; Makuška & Gorochovceva, 2006; Wang et al., 2009) and carboxyalkylation (Felicio et al., 2008; He et al., 2011; Liu et al., 2012; Nguyen et al., 2009). Apart from substitution and chain elongation methods, some researchers studied γ-irradiation in order to reduce the molecular weight of chitosan with minor changes to the structure of the chitosan (Yoksan et al., 2001). Among the various modified chitosan, only a few has been applied as the polymer electrolyte base (Rosli et al., 2012; Winie &

Arof, 2006; Winie et al., 2009). Hexanoyl chitosan based polymer electrolyte achieved a conductivity value of 4.26×10ˉ5 S cmˉ1 with lithium trifluoromethanesulfonate, LiCF3SO3 (Winie et al. 2009).

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Figure 2.5: Multifaceted derivatization potential of chitin/chitosan (Prashanth &

Tharanathan, 2007).

2.1.3. N-phthaloylation of chitosan

In the previous section, we have seen that there are numerous methods to modify chitosan. As discussed before, chitosan is insoluble in organic solvents due to the hydrogen bonds between the amino and hydroxyl groups with the solvents. In this work, the solubility of chitosan in organic solvents has been improved with phthaloylation as shown in Figure 2.6.

Chitin/Chitosan (Crustacean waste, Fungi)

Derivative

Subtitution

O/N- carboxyalkylation

Acylation

Sulfation

Schiff's Base

Enzymatic subtitution

Metal chelation

Chain elongations

Cross-linking

Graft copolymerisation

Polymer network

Depolymerization

Chemical

Acids

Free radicals

Physical

Radiation

Ultrasound

Microwave

Enzymatic

Chitinase/

Chitonase

Non- specific enzyme (lipase, protease, lysozome)

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O O

O OH

HO

HO O

HO NH2

NH2

O O

O OH

HO

HO O

HO NH

NH2 O

CH3

O O

OH

HO NH2

Chitosan n

O O

O

DMF

O O

O OH

HO

HO O

HO O O

O OH

HO

HO O

HO NH

N

O CH3

O O

OH

HO

n N-phthaloylchitosan

O N O

N O O

O O O N O

Figure 2.6: Phthaloylation of chitosan

In 1991, Nishimura and colleagues have replaced the two hydrogens of the amino group with a hydrophobic phthaloyl group to destroy the inherent crystalline structure, thereby improving solubility of chitosan in general organic solvents. This site selectivity reaction which introduced side substituent at predetermined positions of the sugar rings, occurs at the primary amino group of the C-2 position, primary hydroxyl at C-6 position and secondary hydroxyl functionalities at C-3 position has distinctly different reactivities (Kurita et al., 2000; Rout et al., 1993; Yoksan et al., 2001). The structure of phthaloylchitosan is shown in Figure 2.6. Phthaloylchitosan shows excellent solubility in organic solvents such as DMF, DMSO, DMAc, and pyridine (Bian et al., 2009; Kurita et al., 2007; Kurita et al., 2000; Kurita et al., 1998; Kurita et al., 1993; Liu et al., 2005;

Nishimura et al., 1991).

Phthaloylation can be obtained by refluxing chitosan with phthalic anhydride in a particular solvent for 5-7 hours at temperature greater than 100 ℃ (Kurita et al., 1998;

Kurita et al., 1993; Nishimura et al., 1991). Besides, phthaloylchitosan can also be successfully obtained by microwave radiation under nitrogen atmosphere (Liu et al., 2005; Liu et al., 2004). Another method to prepare PhCh is by preparing gel-like chitosan with precipitation of an aqueous acetic acid chitosan solution into aqueous NaHCO3, followed by subsequent multiple solvent replacements with DMF (Rout et al., 1993).

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Product for phthaloylation of chitosan will usually produce a little O-phthaloylation other than N-phthaloylation as shown in Figure 2.7 (Kurita et al., 2001; Liu et al., 2004).

O

O O

O

HO O O

O O

HO O

HO

NH

N

O CH3

n

N O

O

O O

O O

HO

O O

OH

O

O O

HO

Figure 2.7: Structure of N- and O-phthaloylchitosan.

Another method to avoid O-phthaloylation occurring and thus resulting N- phthaloylation only can be achieved by adding in a small amount of hydroxy-containing compounds such as ethanol, water, ethylene glycol or methyl cellosolve into the solvent for reaction. However, the product exhibits low solubilty in common organic solvents as it swelled in DMF, DMSO and pyridine and high degree of crystallinity as shown in Figure 2.8. The product of this method conflicts with the original purpose of this project and is less suitable as a base polymer electrolyte. Therefore, PhCh was synthesized by the conventional method.

From all of the practical viewpoints of protection, deprotection and solubilisation, phthaloylation is particularly attracted at amino group of chitosan (Kurita et al., 2007).

N-Phthaloylation is commonly used in synthesis process as an intermediate step as it can easily be deprotected to generate free amino group (Kurita et al., 2000; Wang et al., 2009;

Yoksan et al., 2001; Yoksan et al., 2004). Phthaloylated chitosan (PhCh) is thus a suitable precursor for a variety of site-specific and quantitative modification reactions to construct well-defined molecular environments on chitosan. Some of the synthesis works that involved PhCh as the key precursor are listed in Table 2.3.

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Figure 2.8: X-ray diffraction diagrams of (A) fully deacetylated chitosan, (B) PhCh prepared in DMF and (C) PhCh prepared in DMF/water (95/5) (Kurita et al., 2001).

Table 2.3: Applications of phthaloylchitosan.

Chitosan derivatives Structure Ref.

Regioselective introduction of α- mannoside branches at C-6 of chitin and chitosan.

O O

HO

NHAc O

OH

HO HO

OH

n

Kurita et al. (1998)

Regioselective introduction of N- acetyl-D-glucosamine and D-glucosamine branches at C-6 of chitin and chitosan

O O HO

NHAc q O

O HO

NHAc OH

O O HO

NHAc p

OH Kurita et

al. (2000) Chain

modification of γ-ray irradiated chitosan under the conditions where reaction occurs mainly at hydroxyl groups

O O

O OCNH(CH2)5CH3

H3C(H2C)5HNCO

HO O

HO

NHCOCH3

NH2

O O

OCNHNH2

HO NH2

O

H3C(H2C)5HNCO O HO

NHCOCH3

n m

O

O

O

O

Yoksan et al. (2001)

Introduction of β- maltoside branches at the C-6 position of chitin and chitosan.

O HO O

NHAc n O

O O

HO OH O OH

HOHO OH

OH Kurita et

al. (2003)

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Table 2.3 continued.

Chitosan derivatives Structure Ref.

N-acylation of chitosan with the quaternary betaine moiety.

O HO O

HN n

OH

N O

Cl

Holappa et al. (2004)

Synthesis of Chitosan-O-PEG- galactose

O O

O ROH2C

CH2OR HO

HO O

HO

CH2OR

NH2

H

n R =

O HO

OH HO

OH

O(OCH2CH2)m CH2CH2

Lin & Chen (2007)

Selective C-6

oxidation of chitosan by 2,2,6,6-tetrameth- ylpiperidine-1-oxy radical

O HO O

N n

OH

H3C CH3

H3C

Bordenave et al. (2008)

Synthesis of chitosan- g-poly(N-

isopropylacrylamide)

O OH

HO

NH2 O

O O

HO

NH2 O

O O

OH

HO

NHCCH3 O C

CH O

O C OH H2C PNIPAAm

c d n

Mu & Fang (2008)

Synthesis of 6- N,N,N-

trimethyltriazole

chitosan O

HO O

NH2 n N N N

N Br

Gao et al.

(2009)

The usefulness of the PhCh used as a precursor or intermediate in the synthesis of chitosan derivatives can be observed from Table 2.3. However, PhCh is rarely used as an end product except for self-assembled polymeric micelles (Casettari et al., 2012).

Other workers have also used PhCh for the same purpose i.e. N-phthaloyl chitosan-g- mPEG (Opanasopit et al., 2006), N-phthaloyl-carboxymethylchitosan (Peng & Zhang, 2007) and N-phthaloylchitosan-g-polyvinylpyrrolidone (Bian et al., 2009). Due to the presence of lone pair electrons on the oxygen of carbonyl (C=O), –N– and hydroxyl (–

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OH) groups in the structure of PhCh, it has the potential to become a base for polymer electrolytes. Thus, in this study, the potential of this biopolymer chitosan as an electrolyte in dye-sensitized solar cell is investigated.

2.2. Solar cell

A solar cell is a device that converts light energy directly into electrical energy through the process of photovoltaic. Solar energy conversion is considered the most credible and viable way to face the growing energy demand, both for its high intensity (1000 W m-2 at ground level with the sun directly overhead) and equitable geographical distribution (Bella et al., 2014). The general understanding of how solar cells work is that sunlight is composed of photons with a spectrum of energies. Photons can interact with atoms. With enough energy the photons release an electron from the atom. For solar cells to produce electricity, it must be able to “collect” the electron once separated from the atom. The electrons flow is the photocurrent.

Solar cells can be divided into several types. According to their material composition, these can be silicon solar cell, perovskite solar cell, cadmium telluride solar cell, quantum dot solar cell, plasmonic solar cell, multi-junction solar cell and dye sensitized solar cell.

2.2.1. Dye-sensitized Solar Cell (DSSC)

Out of the various kinds of solar cell, DSSC has been widely studied. DSSC have many advantages, namely cheap fabrication without expensive and energy-intensive high-temperature and high vacuum processes and compatibility with flexible substrates.

DSSC can be presented in various looks in order to facilitate market entry, both for domestic devices and in architectural or decorative applications (Grätzel, 2005).

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A DSSC consists of three main parts as illustrated in Figure 2.9 which are:

a) Photo-active electrode b) Counter electrode c) Electrolyte

Figure 2.9: Dye-sensitized solar cell configuration

There are three important steps for photocurrent generation (Mohamad et al., 2007; Park, 2010) in DSSCs as shown in Figure 2.10.

Figure 2.10: Steps for generation of photocurrent in DSSCs

Cations and anions (usually a free iodides) are formed when the salt(s) dissociate.

The iodide, Iˉ will interact with I2 (also added in the electrolyte) form a triiodide according to the equation 2.0.

Charge Generation Charge Separation Charge Collection Conductive glass substrate

Conductive glass substrate

Compact layer

Nanocrystalline semiconductor

Electrolyte

Pt coated

Photo-active electrode

Electrolytes Counter electrode

Dye/Sensitizer

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𝐼+ 𝐼2 → 𝐼3 (2.0)

Charge is generated when the dye (S*) in DSSC absorbs photons that excite the ground state (or Highest Occupied Molecular Orbital, HOMO) electrons of dye to the excited state (Lowest Unoccupied Molecular Orbital, LUMO) as shown in Equation 2.1.

The photo-excited electrons are separated from the oxidized dye when they are injected into the mesoporous TiO2 that occurs within pico- to femto-seconds.

𝑆(𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑 𝑜𝑛 𝑇𝑖𝑂2) → 𝑆+(𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑 𝑜𝑛 𝑇𝑖𝑂

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